As the manufacturing of paper has improved over time, a major factor in increasing the cost effectiveness with which paper is manufactured has been the increase in machine speed. A critical problem with increased machine speed is the difficulty in increasing drying speed without increasing the number of dryers proportional to the higher machine speeds. A major advance in increasing the dryness of the web as it leaves the pressing section of the papermaking machine was achieved through the introduction of extended nip presses.
An extended nip press utilizes a shoe which is forced against a backing roll. A conventional press roll will have a nip of no more than about one to two inches in width, while an extended nip will have a width of about ten inches.
Extended nip presses can increase the dryness of the web with fewer nips, thus resulting in a shorter papermaking machine. A shorter papermaking machine occupies less space and generally has fewer components thereby contributing to lower costs.
Extended nip presses can also contribute to enhancing the bulk properties of the paper and the surface finish of the final web. To get maximum control over the affect which the extended nip press has on the web, it is desirable to be able to control the shape of the pressure profile the web is subjected to as it moves through the nip formed between the shoe and the backing roll. In order to gain better contact over the extended nip the shape of the shoe may be varied and the shoe can be supported on two spaced apart pistons. But with greater controllability comes greater complexity and the possibility of instabilities in the control system.
What is needed is a control system for controlling the forces generating the pressure profile of an extended nip press which has inherent simplicity and reliability.